Charles Fabry and Alfred Perot invented the Fabry-Perot interferometer in the late 1800's. The Fabry-Perot interferometer includes two glass plates that have been lightly silvered on facing surfaces. The glass plates are arranged parallel to each other so that the lightly silvered surfaces produce an interference cavity defined by a separation distance between the glass plates. If the separation distance is fixed, the Fabry-Perot interferometer is referred to as a Fabry-Perot etalon.
In either the Fabry-Perot interferometer or the Fabry-Perot etalon, the interference cavity causes multiple beam interference. The multiple beam interference occurs when first and second partially reflecting surfaces are oriented parallel to each other and illuminated by light. Provided that reflection coefficients for the first and second partially reflecting surfaces are not small, the light reflects between the two partially reflecting surfaces multiple times. This produces a transmitted multiple beam interference for the light exiting the second surface in a forward direction and a reflected multiple beam interference for the light exiting the first surface in a reverse direction.
If the Fabry-Perot interferometer is illuminated by a broad light source and the transmitted multiple beam interference is collected by a focusing lens, a circular interference pattern is produced on a screen at a focal length of the focusing lens. The circular interference pattern exhibits bright narrow rings of light separated by larger dark rings.
Goossen et al. in “Silicon modulator based on mechanically-active anti-reflection layer with 1 Mbit/sec capability for fiber-in-the-loop applications,” IEEE Phtonics Technology Letters, Vol. 6, No. 9, September 1994, pp. 1119–1121, teach a mechanical anti-reflection optical switch. The optical switch consists of a SiNx membrane suspended over a Si substrate. The SiNx membrane has a square shape and is suspended from corners by arms. The SiNx layer has a thickness of a quarter wavelength of incident light. A SiNx index of refraction for the SiNx layer is a square root of a Si index of refraction for the Si substrate. When an air gap separating the SiNx membrane from the Si substrate is an even multiple of a quarter wavelength, an antireflection condition exists. When the air gap is an odd multiple of a quarter wavelength of the incident light, a high reflection condition exists. The optical switch is in an off-state when the anti-reflection condition exists and is an on-state when the high reflection condition exists.
Fabricating the SiNx membrane so that the SiNx index of refraction is the square root of the Si index of refraction is difficult. Further, fabricating the arms and the SiNx membrane in a reproducible manner so that production devices operate in a similar manner is difficult. Moreover, it is desirable to have an optical switch which is more economical to produce than the optical switch taught by Goossen et al.
Miles, in U.S. Pat. No. 5,835,255 issued on Nov. 10, 1998 and entitled, “Visible Spectrum Modulator Arrays,” teaches a micro-fabricated interferometric light modulator. The micro-fabricated interferometric light modulator includes a transparent substrate and a rectangular membrane suspended above the substrate. The transparent substrate includes first and second surfaces, and also includes a transparent film on the second surface. The transparent film is conductive. A mirror, either a metal or dielectric mirror, lies on the transparent film. The membrane is suspended above the mirror by parallel support structures, which support two edges of the rectangular membrane. The membrane is both reflective and conductive. The membrane and the mirror form an interferometric cavity which is modulated by biasing the membrane relative to the transparent film. In operation, the micro-fabricated light modulator modulates light incident upon the first surface of the transparent substrate by interferometrically causing the incident light to exit the first surface or by interferometrically causing the incident light to not exit the first surface.
Miles further teaches an alternative micro-fabricated interferometric light modulator in which the membrane is a square membrane. The square membrane is suspended by arms from centers of each of four lengths defining the square membrane.
Fabricating the transparent and conducting film of the micro-fabricated light modulators is difficult. Further, keeping a separation distance defining the interferometric cavity of the micro-fabricated light modulators constant across the interferometric cavity is difficult. Additionally, the combination of the rectangular membrane and the parallel support structures gives rise to a tendency for the rectangular membrane to deform cylindrically. The cylindrical deformation of the rectangular membrane reduces the effectiveness of the interferometric cavity. Moreover, it is desirable to have an interferometric light modulator which is less costly to manufacture and which is more reproducible than the micro-fabricated interferometric light modulators taught by Miles.
What is needed is an interferometric light modulator which is economical to fabricate, which is more easily reproducible in a production setting, which does not rely on a rectangular membrane supported by parallel support structures, and which does not rely on arms to support a moving surface.